1. Field of the Invention
This invention generally relates to an optical pick-up for use in a optical recording and reproducing system, such as an optical disc system, and, in particular, to an optical pick-up using a multiple of beams.
2. Description of the Prior Art
An optical disc system in which a laser beam from a semiconductor laser is converged into a tiny spot onto an optical disc by an objective lens to carry out recording, reproduction and erasure of information to and from the optical disc has recently attracted much attention. This is because such an optical disc system has various advantages, such as a large capacity and a low cost per bit.
However, it is not free of disadvantages one of which is a relatively long processing time as compared with hard discs, such as magnetic discs. This is because, in the case of recording on an optical disc, in response to a command, an access is made to an optical disc to thereby locate a tiny laser spot at a desired address location of the disc and then information is recorded on the disc. Upon completion of recording, the information thus recorded is then reproduced for confirmation. Accordingly, a recording mode of such an optical disc is a two step operation, including "recording" and "reproduction for confirmation."
In order to cope with this situation, an optical pick-up using a multiple of laser beams has been proposed. That is, in accordance with this proposal, a plurality of laser beams are focused onto the same recording track of an optical disc spaced apart from each other and one of the beams is used for recording information on the recording track while another beam is used to reproduce the information thus recorded immediately after its recording to confirm the recorded information. With this structure, since recording and reproduction for confirmation of the recorded information can be carried out substantially at the same time, the time required for a recording mode can be minimized. A typical optical pick-up of the type using a multiple of beams is discussed, for example, in Japanese Patent Post-examination Publication No. 57-60697.
FIG. 3 illustrates an example of an optical pick-up using two laser beams. As shown, there is provided a semiconductor laser 1 including a pair of light-emitting points lA and lB for emitting a pair of laser beams A and B, respectively. These light beams A and B are collimated by a collimator lens 2 and the thus collimated laser beams enter into a beam splitter 3 where the entering laser beams are deflected toward an optical disc 4. The laser beams A and B are then converged into tiny laser spots 4a and 4b, respectively, on the optical disc 4 by means of an objective lens 5. In the illustrated case, the spot 4a is a leading spot for use in recording of information. Thus, during recording mode, the beam A itself is modulated in accordance with information to be recorded. On the other hand, the other beam 4b is a following beam spot which follows the leading spot 4a on the same recording track for use in confirmation of recorded information.
The light reflected from the optical disc 4 again passes through the objective lens 5 in the opposite direction and enters into the beam splitter 3 where the reflected light from the optical disc 4 is separated from the incident light from the collimator lens 2 and directed toward a detection optical system while being converged by a first convergent or focusing lens 6. The beams A and B are converged at different locations by the first convergent lens 6. That is, the recording beam A is first converged by the convergent lens 6 and then again converged by a second convergent lens 7 to be directed toward a two-part light receiving element 8 for use in focus error detection. A prism mirror 9 serving as a knife edge is locate in the optical path of the second convergent lens 7, so that a focusing error detecting operation is carried out based on a well-known knife edge method. On the other hand, a part of the light passing through the second convergent lens 7 is reflected by the prism mirror 9 toward a light-receiving element 10 for use in tracking error detection. The detection of this tracking error signal is carried out by a well-known push-pull method. Focusing and tracking error signals thus obtained are then supplied to a servo control system of the optical pick-up, in particular objective lens 5.
The laser beam B converged by the first convergent lens 6 is reflected by a prism mirror 11 disposed at a focal point of the first convergent lens 6 and the thus reflected light passes through a third convergent lens 12 and then impinges upon a light-receiving element 13 so that a reproduction signal for confirmation is obtained.
In such an optical disc system, since a servo signal is obtained based on the beam A for recording information on the optical disc 4, it is said that the tracking and focusing errors of the recording laser spot are minimized and thus the reliability of an information pit formed on the optical disc 4 by the optical pick-up is increased. However, there is a chance that such tracking and focusing errors could occur for the beam B for use in confirmation of recorded information. For example, when a tracking servo is to be applied by the recording beam A, even if information is recorded properly, there is a change that a tracking error could occur for the confirmation beam B, in which case an error may be present in the signal reproduced for confirmation. As a result, the reliability of the overall operation of the recording mode of the optical pick-up shown in FIG. 3 could be reduced. Such tracking errors could take place, for example, due to differences in radius of curvature between the inner and outer recording tracks, due to disturbances of recording tracks, and due to shifting of the confirmation beam B from a recording track to which information has been recorded by the preceding recording beam A owing to eccentricity of the optical disc.